1. Technical Field of the Invention
The present invention relates generally to an internal combustion engine control system, and more particularly to a fail-safe system for automotive engine control designed to ensure execution of given control tasks in the even of a failure of a mechanism working to produce triggers for initiating the given control tasks.
2. Background Art
Japanese Patent First Publication No. 2000-104619 discloses an internal combustion engine control system which works to define a fraction of an output interval of first and second cam signals from first and second cam sensors as a control task trigger interval and estimate an acceleration or deceleration of the engine to correct the control task trigger interval with suitable weighting to determine a dummy control task start time instead of a control task start time defined by a crank signal for initiating a control task such as fuel injection control or ignition timing control if a failure of a crank sensor has occurred.
A typical cam sensor installed in an automotive internal combustion engine is constructed to output a cam angular position signal one time every combustion cycle of each cylinder of the engine. The above described engine control system is designed for a V-type four-cycle eight-cylinder engine. The first and second cam sensors output the first and second cam signals at an interval of 90xc2x0 crank angle (CA). In a case of a four-cycle four-cylinder engine, cam signals are outputted at an interval of 180xc2x0 CA. It is, thus, difficult for the above prior art engine control system to estimate the degree of acceleration or deceleration of the engine accurately using the cam signals outputted at such a long time interval. Therefore, even if the control task trigger interval is corrected with suitable weighting to determine the dummy control task start time, it is possible that when the engine is accelerated rapidly, the establishment of the dummy control task start time may be too late. This will result in a difficulty in executing the control task a given number of times within a desired angular interval of revolution of the engine.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide an engine control system which is capable of ensuring execution of a given control task within a desired angular interval of revolution of the engine in the even of a failure of a mechanism working to produce a trigger for initiating the control task.
According to one aspect of the invention, there is provided a control apparatus for an internal combustion engine. The control apparatus comprises: (a) a crank sensor responsive to rotation of a crank shaft of an internal combustion engine to output a crank signal at a first angular interval of the rotation of the crank shaft; (b) a cam sensor responsive to rotation of a cam shaft of the engine to output a cam signal at a second angular interval of the rotation of the cam shaft which is a given multiple of the first angular interval of the rotation of the crank shaft; (c) a crank sensor failure detecting circuit detecting a failure of the crank sensor to provide a failure signal indicative thereof; (d) a control circuit executing a given control task cyclically in synchronism with rotation of the engine; and (e) a control task start time defining circuit working to define a crank signal-triggered control task start time at which the given control task is to be initiated cyclically in the control circuit as a function of an interval between sequential inputs of the crank signals from the crank sensor. If the crank sensor has failed, the control task start time defining circuit is responsive to the failure signal from the crank sensor failure detecting circuit to define a cam signal-triggered control task start time at which the given control task is to be initiated every input of the cam signal from the cam sensor and also to define a given fraction of an interval between sequential inputs of the cam signals as a dummy crank signal-triggered control task start time interval at which the given control task is to be executed following the input of the cam signal. If the cam signal is inputted before the number of times the control task is to be executed cyclically at the dummy crank signal-triggered control task start time interval is not yet reached, the control task start time defining circuit produces at least one trigger to initiate the given control task following execution of the given control task upon the input of the cam signal. This ensures the stability of an operating condition of the engine in the even of the failure of the crank sensor.
If the cam signal is inputted before the number of times the control task is to be executed cyclically at the dummy crank signal-triggered control task start time interval is not yet reached, the control task start time defining circuit may produce triggers in response to the input of the cam signal to initiate the given control task the same number of times as that the given control task is not yet executed at the dummy crank signal-triggered control task start time interval. This achieves execution of the control task a required number of times between two consecutive outputs of the cam signals to ensure the stability of an operating condition of the engine in an emergency running mode.
The control task start time defining circuit may define the dummy crank signal-triggered control task start time interval only when the speed of the engine is less than a given value.